The present invention relates to a method and apparatus for conveying fluids. The invention is particularly useful in conveying liquid-gas mixtures and is therefore described below with respect to these applications, but it will be appreciated that the invention could advantageously be used in many other applications, such as in conveying liquid-sand mixtures, gas-liquid mixtures, viscous liquids, and immiscible-liquid mixtures. Also, while the invention is particularly useful in oil production wells, it can also be used in gas and condensate production wells, liquid transporting systems, wells drilling systems, etc.
Free-flowing oil wells utilize the natural energy of the underground reservoir, including the reservoir pressure and the energy of gas dissolved in the oil, for lifting the oil from the underground reservoir to the surface. However, this natural pressure is continuously depleted during the operation of the oil well, so that an artificial lift is required if the oil well is to continue to produce. Artificial lift may be provided by one of many known pumping methods, such as by using the rod pump or centrifugal submerged pump, by injecting gas into the well, or by including different kinds of gas-lift and plunger-lift methods, as described for example in the U.S. Pat. Nos. 5,105,889 and 5,562,161.
An important object of the present invention is to enhance the delivery of oil through an oil well in order to increase the production and/or reduce the production cost of the oil well, and/or to delay the point where an artificial lift becomes necessary.
While the invention is particularly useful in oil wells, the invention may also be advantageously used in many other applications involving the conveying of a fluid, especially a liquid-gas mixture such as oil flowing through a horizontal pipeline. Accordingly, another object of the invention is to provide a method and apparatus for conveying a fluid flowing through a tubing
According to a broad aspect of the present invention, there is provided a method of conveying a fluid flowing through a flow passageway in a tubing, comprising: introducing into the flow passageway zones of small cross-sectional area alternating with zones of large cross-sectional area to produce high-velocity, low-pressure zones alternating with low-velocity, high-pressure zones; and providing abrupt transitions from the small cross-sectional area zones to the large cross-sectional area zones to produce a turbulent flow generating swirls and eddies at such transitions.
The invention is particularly useful for conveying liquid-gas mixtures, in which case the turbulent flow produced at the abrupt transitions is effective to intensively mix the liquid and gas, and thereby to produce small and uniformly-distributed gas bubbles such as to decrease the density of the flowing liquid-gas mixture and to enhance its flow through the passageway.
Thus, as the liquid-gas mixture flows through the small area zones, the flow velocity increases and the pressure decreases according to Bernoulli""s Law. Where the pressure at the entrance to the small area zone is higher than the bubble-point pressure inside the small area zone, the pressure becomes lower than the bubble-point pressure. This causes early (compared to regular well construction) gas liberation from the oil, and therefore early utilization of its energy. On the other hand, if the pressure at the entrance to the small area zone is lower than the bubble-point pressure, a heterogeneous liquid-gas mixture enters the small area zone, increases its velocity, and reduces the pressure. As a result, any remaining dissolved gas is liberated from the oil, adding its energy to the flow. In addition, the higher velocity provides the primary destruction of the large gas bubbles; it also stabilizes the finely dispersed gas-liquid structure and prevents the coagulation of fine gas particles into larger ones.
On the other hand, during the flow in the large area zones, the velocity decreases and the pressure increases. By providing abrupt transitions from the small area zones to the large area zones, turbulence is created generating swirls and eddies which convert the large heterogeneously-distributed bubbles into small homogeneously-distributed bubbles. In addition, the high velocity jet flow of the mixture in this transition zone also contributes to the destruction of the large bubbles and to the generation of the finely dispersed oil-gas mixture in the turbulent flow. The result, particularly after a series of such alternating transitions, is to reduce the overall density of the liquid-gas mixture as it flows through the passageway.
According to some described embodiments, gradual transitions are provided from the large cross-sectional area zones to the small cross-sectional area zones to reduce the pressure loss produced in the flow passageway.
As further indicated above, the invention is particularly useful in oil wells, in which case the flow passageway is in upwardly-extending tubing of the oil well. In such cases, the flow is enhanced not only by the lower density produced by the presence of uniformly distributed small gas bubbles, but also by the increased bubble-pressure produced by such small uniformly-distributed gas bubbles. However, the invention could also be advantageously used in other applications, such as in oil and gas wells which include horizontal sections, or which include coiled tubing, as well as in oil pipelines wherein the flow passageway is in horizontally-extending tubing.
As will also be described below, the invention could also be used in applications wherein the fluid is a liquid-sand mixture, the high velocity flow produced in the small cross-sectional area zones being effective to carry out the sand with the liquid flow, and the turbulent flow produced at the abrupt transitions being effective to intensively mix sand particles within the liquid, and thereby to enhance the flow through the tubing.
Another application is one wherein the fluid is a gas-liquid mixture comprising liquid droplets within a flowing gas, the high velocity flow produced in the small cross-sectional area zones being effective to carry out the liquid with the gas flow, and the turbulent flow at the abrupt transitions being effective to reduce the size of the liquid droplets and to uniformly distribute them within the gas flow.
The invention could also be applied wherein the fluid is a viscous liquid, the high velocity flow produced in the small cross-sectional area zones being effective to produce high shear stresses in the liquid, the turbulent flow in the abrupt transitions being effective to destroy the internal structure of the liquid, the pressure fluctuations produced in the liquid flowing through the alternating small cross-sectional area zones and large cross-sectional area zones being effective to produce viscosity reductions and liquid restructuring which further improve the hydrodynamic characteristics of the liquid.
According to further features in some preferred embodiments of the invention described below, the alternating zones of small and large cross-sectional areas are introduced into the flow passageway by inserting retrievable inserts within the tubing, which inserts have outer surfaces shaped to define the small and large cross-sectional area zones with the inner surface of the tubing. In other described embodiments, the alternating zones are introduced into the flow passageway by providing the tubing with tubing sections having inner surfaces shaped to define the small and large cross-sectional area zones.
According to another aspect of the present invention, there is provided an oil well operating in accordance with the above method for delivering oil from an underground reservoir to the surface.
Further features and advantages of the invention will be apparent from the description below.